EP0210575B1 - Electrical radiant heating element for heating hot plates, and process and device for its manufacture - Google Patents

Abstract

An electric radiant heater (11) for heating a glass ceramic cooking surface (12) contains a dish-shaped insulator (14), to whose inner bottom surface (18) are fixed heater coils (20). The heater coils have an oval cross-section and are pressed by their narrow sides into surface (18) and fixed. The fixing pressing in takes place during the moulding of the moist insulator (14), made from fibrous material and by means of a male mould receiving the heater coils in slots.

Description

The invention relates to an electric radiant heater for heating heating surfaces and a method and a device for its production.

From DE-OS 27 29 929 a radiant heater has become known in which heating coils with a circular cross section are embedded in the surface of an insulating body which contains fibrous material by being pressed in and thereby fixed. It is also mentioned there that the heating coils can have an oval shape, namely lying flat, i.e. with its smallest extension, i.e. the small axis towards the heating plate. This serves to save overall height.

This type of embedding works excellently as long as the heating coils do not fall below a certain wire thickness and their total length does not become too great in relation to the surface available. For many radiant heaters, for example those operated in a multi-cycle circuit, the required wire lengths are very large and the wire diameter is small, especially for versions with higher voltages (380 V). With the resulting small helix pitches and the lower stability of the helix, a correct fixing is no longer possible.

From FR-A-929 589 a radiant heater has become known in which a resistance wire bent in the form of a sinus curve band is embedded with the apexes of one side of the curve in an initially plastic, hardenable material. This arrangement requires an absolutely rigid fixation in hardened ceramic material, otherwise the band could tip to the side and cause short circuits with neighboring bands. This makes the radiator very thermally and mechanically sensitive and unsuitable for thin resistance wires.

It is an object of the invention to provide an electric radiant heater, the heating coil of which, with a good fixation on the insulating body and with a relatively small proportion of embedded heating resistance surface (degree of embedding), can also be produced with a smaller wire diameter.

This object is achieved by claims 1, 2, 8 and 9.

The embedding of an approximately oval wire helix on one of its narrow sides has several advantages: Given the width of the helix in the direction of the surface, that is, depending on the usable area unit, the oval shape of the wire length is somewhat longer than that of the circular helix and much larger than in the case of lying oval shape. Furthermore, the coil is fixed much better when embedding a certain fraction of its total wire length, because a much larger arc piece is held. For example, when embedding a 180 ° bend make up, for example, only a third or a quarter of the total winding circumference, so that sufficient radiation area remains. Due to the larger total length of the wire-shaped resistance material to be accommodated, the helix pitch, ie the proportion of the gaps between the individual turns, can also be greater, so that penetration into the embedding material is promoted. Above all, the penetration of the more curved side surface into the material is easier to make, and when pressed in, even thin wires do not tend to deform or fall over in the longitudinal direction of the coil and lie flat. This also contributes to the fact that the helix can be guided in a groove when inserted over two thirds or three quarters of its circumference and the pressure acting on the free apex presses the flatter sides of the oval against the groove walls and thereby stiffens the helix, which in the Transverse direction also has a large section modulus. There is no fear of the heating coil tipping to the side because, unlike FR-A-919 589, the embedded wire areas run transversely to the heating coil extension.

Fig. 1

einen schematischen Schnitt durch einen elektrischen Strahlheizkörper und eine Heizplatte,

Fig. 2

einen vergrößerten Schnitt durch ein Detail des Strahlheizkörpers mit Heizwendel,

Fig. 3

einen vergrößerten Querschnitt nach der Linie III-III in Fig. 2,

Fig. 4

ein stark vergrößertes Detail aus Fig. 2 und

Fig. 5 und 6

zwei Herstellungsschritte für den Strahlheizkröper mit zugehöriger Vorrichtung.

This and further features of the invention are evident from the subclaims and the description and the drawing, the individual features being able to be advantageously realized individually or in groups in the form of subcombinations in one embodiment of the invention and also in other fields. Embodiments of the invention are shown in the drawing and are explained in more detail below. Show it:

Fig. 1

1 shows a schematic section through an electric radiant heater and a heating plate,

Fig. 2

an enlarged section through a detail of the radiant heater with heating coil,

Fig. 3

3 shows an enlarged cross section along the line III-III in FIG. 2,

Fig. 4

a greatly enlarged detail from Fig. 2 and

5 and 6

two manufacturing steps for the radiant heater with associated device.

The radiant heater 11 shown in Fig. 1 is used to heat a glass ceramic heating surface 12 or other heating surfaces made of ceramic. Although it is preferably intended for heating one of several hotplates on a coherent cooking surface, it can also be used for single hotplates etc.

The radiant heater 11 contains a sheet metal carrier shell 13, in which a plate-like insulating body 14 with a round bottom 15 and a peripheral edge 16 is arranged like the carrier shell. For better thermal insulation, this mechanically quite firm and manageable insulating body 14 can be underlaid by an insulating layer 17, which is less mechanically strong but has excellent thermal insulation, for example made of pyrogenic silica. The insulating body is pressed with the upper end face of the edge 16 onto the underside of the cooking surface 12 by spring elements, not shown. It preferably consists of a fibrous, high-temperature-resistant insulating material, for example an aluminum oxide fiber, which is commercially available under the name "Fiberfrax". Other mineral fibers or other compressible insulating materials, such as vermiculite, can also be used.

Heating coils 20 made of wire-shaped electrical resistance material 21 are partially pressed and fixed into the substantially flat surface 18 of the bottom 15 of the insulating body 14. Each turn 60 of the heating coil 20 has an approximately oval shape, which can be seen in particular in FIG. 3, which consists of two approximately semicircular arches on the narrow sides of the oval and two connecting them, essentially rectilinear or slightly outward or inward (dash-dotted line in Fig. 3) there are curved sides 24, 25. The more curved narrow sides 22, 23 can, in deviation from the semicircular shape, be slightly more curved in the area of their apex 26. They are produced by deforming an initially tightly wound spiral, that is to say circular to circular, by pressure between two pairs of jaws or rollers into the approximately oval shape and finally by mechanical stretching to the desired length or spiral pitch (left in FIG. 2) detect) and be fixed in this form largely stress-free by annealing with self-heating due to current passage. The type of deformation into the oval shape also results in the shape shown in FIG. 3, which has proven to be very advantageous for the desired purpose. Above all, the resulting stronger, pointed arch-like curvature in the region of the apex 26 facilitates penetration into the insulating body, while the straight sides ensure good guidance in the production tool. However, other oval or oval-like shapes can also be used, which have a clear difference in dimensions between their vertical and horizontal extension. All of these cross-sectional shapes are referred to here as approximately oval.

The insulating body 14 is produced according to the following method: from a slurry of the insulating fibers in water, to which inorganic or organic binders of the usual type are also added, the fibers are sucked in by a suction mold which has a negative in the thickness of the insulating body, but with a free lower one Bottom 29 forms. A resulting thickening on the bottom 29 is cut off and, if necessary after a pre-pressing, the soft, moist insulating body 14 '(FIG. 5) is introduced into a mold 30.

The prepared oval heating coils are inserted over their entire length into grooves 33 in the area of non-distorted ends after welding of connecting pins 28, the shape of which corresponds to the oval shape of the heating coils 20, but has a smaller depth. The oval heating coils are thus inserted into the grooves 33 with an upright cross section and project with about a third to a quarter of their larger cross sectional dimensions a (FIG. 3) over a pressing surface 34 of the punch 32 carrying the grooves. The width of the grooves is such that the heating coils can be inserted well, but are guided as precisely as possible. The grooves 33 run in the case of circular radiators mostly spirally, in single or multi-course spirals or also a double spiral, which has a reversal point in the middle and can consist of one or more spiral sections, each of which is provided with electrical connection pins 28, which are provided with the insulator 14 lead wires 61 are welded.

The insertion die 32 and the mold 30 are moved relatively towards each other in the direction of the arrows 34 and 35 and the insertion die presses the still plastically deformable insulating body preform 14 'into its final shape 14, which can be seen in FIG. 6. The heating coils 20 with their Narrow sides 22 pressed or pressed into the surface 18 and thus the material of the insulating body and fixed.

In particular, if the heating coils consist of very thin resistance material in the order of magnitude between 0.15 and 0.25 mm, this would hardly be possible with round coils. Due to the oval shape and the good guidance in the grooves 33, even greater compressions of the insulating body and thus a good fixing can be achieved. Due to the good guidance, the helixes do not tend to buckle laterally and cannot fall over in the longitudinal direction of the helix because the pressure on the curved section 22 of the helix spreads the sides 24 and 25 somewhat apart and clamps them on the groove inner wall (FIG. 6). A good definition also contributes to the fact that the helix pitch h (FIG. 2) is relatively large compared to the diameter d of the resistance material 21 and thus leaves sufficient space for the fiber material to penetrate between the turns 60 and form a bead 40. Its surface 41 is somewhat below the uninfluenced, flat surface 18 of the insulating body 14, but above the penetrated winding sections 42. These cut into the fibrous material, which at least partially closes behind them. It is advantageous that depressions or channels 43 are formed by the penetration, which are only partially above the penetrated turn section 42 close again. On the one hand, radiation from this part is still possible or heat is dissipated without mediation of the insulating body, and on the other hand, the fibers 44 that cross this winding section 42 form a particularly elastic hold that does not break even during movements due to thermal expansion. Depending on the materials and dimensions used, however, the embedding can be more or less complete and deep and the depressions 43 can also be completely closed, in particular if a less elastic and fibrous material is used. From Fig. 3 it can be seen that the embedding of the coil on the outer edge of each turn is slightly higher than on the inner edge, so that a flat channel forms in the area of the heating coil, which favors the fixing. Above all, however, it can be seen that it is possible to keep the majority of the interior 45 of the insulating material free, so that no heat accumulation can form there, which could lead to premature mechanical and thermal wear of the heating coils.

After the pressing shown in FIG. 6, the insertion die 32 and the mold 30 are removed from one another again, the heating coils remaining in the insulating body. They can be easily pulled out of the grooves 33 because they spring back a little when the punch is relieved and have play with the groove walls. The now pressed, but still moist insulating body 14 is now brought into its relatively solid final state by drying or other hardening measures.

The invention makes it possible to accommodate large lengths of wire on a given unit area, so that even radiators in a multi-cycle circuit with small ones Partial services can be produced. The relatively large ratio between the possible helix pitch h to the wire diameter d of the resistance material 21, preferably h / d = 2 to 10, also contributes to the radiation and ventilation conditions being good. Since, in addition to the extension of the wire diameter for a given width, the oval shape also improves the fixability, it is possible to arrange the spirals in relatively closely spaced spiral paths, so that this also increases the wire length to be accommodated per unit area and enables very uniform heating. The preferred uniform fixing of the heating coils over their entire length also avoids creeping movements of the coil and creates short-circuit protection even at a short distance. However, if it is not a question of a very narrow occupancy, it would also be possible to fix the ribs or warts at a distance from one another as a priority. A value of more than 1.5, preferably approximately 2, has proven to be the preferred ratio between the length of the large axis a of the oval helical cross section directed towards the heating surface 12 and away from the insulating body, in the direction of the small oval axis 51. Usually, the large axis 50 (FIG. 3) is approximately perpendicular to the surface of the insulating body 18. However, given appropriate conditions, it is also possible to set it somewhat obliquely, as long as the advantages achieved by the preferred indentation of the narrow side 22 in the insulating body are retained can be. The good guidance and stability of the heating coil and the "cutting effect" of the relatively thin usable wires also make it possible to press-fit into insulating materials whose resistance to penetration is relatively high and can be used after pressing without hardening or drying process. This includes, in particular, granular insulation materials.

The preferred values of the ratio helix pitch / wire diameter (warp) h / d can change depending on the axis length ratio a / b of the oval helix cross section and the absolute wire diameters. It has been determined with an insulating body made of Fiberfrax that good fixing ratios for a wire diameter d = 0.25 mm at a / b = 2 and h / d = 2.5 or above have been achieved, the minimum values for h / d being larger a / b could decrease and should increase with smaller a / b (e.g. for a / b = 1.5 to h / d = 3). In contrast, smaller wire diameters enable larger h / d values, e.g. d = 0.18; a / b = 2; h / d over 3. These ratios can change depending on the fiber length, absorbance, fiber quality, binder content etc. of the insulating body.

Because of the relatively large press-in depth compared to the helix diameter, tolerances in the helix dimensions and in the press-in depth have a much smaller effect.

Claims (11)

1. Electric radiant heater for heating heating surfaces (12), particularly glass ceramic hotplates, with an insulator (14) made from electrically and thermally insulating material, in whose surface (18) is partly embedded in an area of more marked curvature at least one heating resistor (20) comprising a wire bent with a non-uniform curvature and the greater transverse extension of the heating resistor (20) is directed away from the surface (18) of the insulator (14), characterized in that the heating resistor (20) is a heater coil with numerous, spaced turns of electric resistance material (21), which has a roughly coil cross-section with a longer and a shorter axis (50, 51) and with in each case two facing sides (22, 23; 24, 25) with a greater and lesser curvature up to an optionally linear configuration and that the embedded wire areas (42) extend roughly at right angles to the longitudinal extension of the heating resistor (20) and that the insulator (14) is mainly made from fibres (14) bound to form a relatively rigid body and preferably the resistance material (21) in the embedded portion (42) of the fibres (44) of the insulator (14) engaging in the coil interior, but the remaining coil interior (45) is substantially free from insulating material.
2. Electric radiant heater for heating heating surfaces (12), particularly glass ceramic hotplates, with an insulator (14) made from electrically and thermally insulating material, in whose surface (18) is partly embedded in an area of more marked curvature at least one heating resistor (20) comprising a wire bent with a non-uniform curvature and the greater transverse extension of the heating resistor (20) is directed away from the surface (18) of the insulator (14), characterized in that the heating resistor (20) is a heater coil with numerous, spaced turns of electric resistance material (21), which has a roughly coil cross-section with a longer and a shorter axis (50, 51) and with in each case two facing sides (22, 23; 24, 25) with a greater and lesser curvature up to an optionally linear configuration and that the embedded wire areas (42) extend roughly at right angles to the longitudinal extension of the heating resistor (20) and that the insulator (14) comprises a granular insulating material usable after compressing without a hardening process and preferably in the embedded portion (42) the resistance material (21) is overengaged by parts of the insulator (14) engaging in the coil interior, but the remainder of the coil interior (45) is substantially free from insulating material.
3. Radiant heater according to claims 1 or 2, characterized in that the oval coil cross-section comprises two roughly semicircular sides (22, 23), which are preferably more curved in the apex region and sides (24, 25) connecting the same having a substantially linear configuration of the resistance material (21) and preferably the embedding on the outsides of the heater coil (20) extends up to the sides (24, 25) with a substantially linear configuration of the resistance material (21).
4. Radiant heater according to one of the preceding claims, characterized in that the embedding on the entire length of the heater coil (20) is substantially uniform for each turn (60), but only over a portion (42) taking up a fraction of the turn circumference.
5. Radiant heater according to one of the preceding claims, characterized in that in the embedded portion (42) located pressed-in depressions (43) running along its turn configuration and which are partly open towards the surface (18) of the insulator (14) and/or the turns (60) of the resistance material (21) are further surrounded by the material of the insulator (14) on the outsides thereof than in the coil interior (45).
6. Radiant heater according to one of the preceding claims, characterized in that several different heater coils (20) are embedded in spiral paths in an insulator (14).
7. Radiant heater according to one of the preceding claims, characterized in that the ratio of the major to the minor axis (50, 51) of the oval coil cross-section is larger than 1.5 and is preferably approximately 2 and/or the ratio of the coil pitch (h) to the diameter (d) of the resistance material (21) is greater than 2 and is preferably between 4 and 8.
8. Process for the production of an electric radiant heater, in which a heating resistor with curved portions is pressed into the surface (18) of a plastically deformable insulator (14), characterized in that the heating resistor is a heater coil (20) with an approximately oval helical cross-sectional shape, which is pressed in with its narrow side (22), for which purpose a circular heater coil under lateral pressure is compressed in the oval turn cross-sectional shape, is expanded axially, held over more than half the turn circumference with upright coil cross-section and is pressed by the free, unsecured part into the surface (18) of the plastically deformable insulator (14) mainly comprising fibres or a subsequently not hardened or dried granular insulating material.
9. Process according to claim 8, characterized in that pressing takes place into the moist insulator (14), mainly made from insulating material fibres mixed with binders and after pressing in the heater coil (20), the insulator (14) is hardened, optionally by drying.
10. Process according to one of the claims 8 or 9, characterized in that on pressing in the heater coil (20), the insulator (14) is pressed into its final shape.
11. Apparatus for producing an electric radiant heater, characterized by an insert male mould (32), which has at least one preferably spiral slot (33) for receiving a roughly oval heater coil (20) with upright coil cross-section, which is sufficiently deep to receive more than half and advantageously more than 60% of the heater coil dimensions (a) in the direction of the larger oval axis (50) and laterally guides the same.

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